Implementing the concept of a variable efficiency of use of individual AA (AA_ef) in feeding systems requires knowledge on the effect of net energy of lactation (E) in addition to the effect of metabolizable protein supply (P expressed as PDI, INRA). Therefore, AA_ef were determined at 2 levels of E (LE: 24.8 vs. HE: 32.6 Mcal/d of NEL) and 2 levels of P (LP: 1,261 vs. HP: 2,227 g/d of PDI) using a 4 × 4 Latin square with 4 cows and 2-wk periods. Individual AA_ef was calculated as the sum of milk protein yield (MPY), endogenous fecal (EFP) and scurf proteins (INRA, 2018) multiplied by their respective AA profile (Lapierre et al., 2016; CNC: 205) and divided by the AA supply minus the AA endogenous urinary losses (AA_S-EU). Mammary net uptake (MU) of AA (6 samples in 12 h) was determined on d 13 of each period. Data were analyzed by MIXED procedure (SAS) with cow as random effect. Increasing E and P supply increased MPY, essential AA (EAA) MU and only increasing E supply tended to increase non-EAA (NEAA) MU. Increasing E supply was partially achieved by increased DMI (14.5 vs. 19.1 kg/d) increasing EFP. Therefore, the AA_ef increased with increasing E supply but at a higher extent in LP diets. The gain in AA_ef with increasing E was mainly explained through a change in the partition of AA toward MPY and EFP at whole body level and of MU AA toward MPY as suggested by the lower EAA MU: milk output ratio. The decreased AA_ef with increased P supply was related to a decreased NEAA and EAA MU relative to the AA_S-EU and to an increased MU: milk output ratio of group 2 AA (Lys, Ile, Leu and Val), both suggesting a higher AA catabolism (Table 1). Table 1.

In mol/12h of N	Diet				SEM	P-value
LELP	HELP	LEHP	HEHP	E	P	E × P
AAS-EU	7.4	7.1	13.6	13.8	0.5	0.76	<0.01	0.18
MPY	3.7	4.2	4.1	5.0	0.2	<0.01	<0.01	0.15
EFP	1.1	1.4	1.0	1.5	0.1	<0.01	0.68	0.02
Uptake
EAA	2.3	2.6	3.0	3.3	0.1	0.02	<0.01	0.92
NEAA	1.3	1.5	1.2	1.4	0.2	0.10	0.40	0.81
AAeff
EAA	0.74	0.92	0.44	0.54	0.01	<0.01	<0.01	<0.01
NEAA	0.68	0.84	0.40	0.50	0.01	<0.01	<0.01	<0.01
MU: milk output
EAA	1.30	1.26	1.51	1.38	0.04	0.06	0.01	0.28
Group 11	1.08	1.02	1.01	1.03	0.03	0.44	0.31	0.15
Group 22	1.23	1.28	1.50	1.34	0.08	0.45	0.06	0.18

¹His, Met, Phe, Tyr, and Trp. ²Lys, Ile, Leu, and Val.